It is well known that dissolution of metal values, in particular gold, from certain types of source materials such as pyrite, arsenopyrite, silica and other refractory types of materials is enhanced by milling the material to smaller particle sizes followed by subsequent leaching using an appropriate leach liquor, in the case of gold, usually a cyanide leach liquor. This is due to the fact that a portion of the metal, particularly gold, which is physically encapsulated is released and made available for leaching. The finer the source material is milled, the more of such metal is released for leaching. Clearly, there comes a point where, due to the cost of milling, it is uneconomical to mill the source material any finer.
In spite of the fact that simultaneous grinding and leaching has been proposed previously (see U.S. Pat. No. 4,269,808 to Kawabata) it has not been possible heretofore to economically treat certain materials, in particular certain precious metals containing materials, by grinding them to sizes substantially finer than 7.9.times.10.sup.-4 inches (20.mu.m). This is evidenced by the large amounts of material worldwide that have been discarded from conventional treatment plants and that could form economically viable feedstock if a suitable recovery process could be found.
It has now surprisingly been found that if milling and simultaneous leaching is effected under certain conditions, enhanced results are achieved.